1. Field of the Invention
The present invention relates to Fresnel lenses and to injection molds used when manufacturing Fresnel lenses. In particular, the present invention relates to a Fresnel lens with improved appearance quality when manufactured by plastic injection molding using a mold and to an injection mold.
2. Description of the Related Art
A Fresnel lens has a zonal structure formed by cutting out lens surfaces and concentrically arranging them on a plane, and can be considered as an aggregate of prisms. Generally, in order for a lens to have sufficient refractive power, the curvature of the lens surface needs to be increased and the lens consequently becomes thicker. However, by making the lens into a Fresnel lens, the lens can have a focusing function while having a flat structure, thereby allowing for compactness and high integration properties. Practical examples of Fresnel lenses include rear-projection screens, condensing lenses in overhead projectors, and focusing screens for viewfinders of cameras.
A common Fresnel lens has spherical lens surfaces and zones that are gradually increased in height from the central zone towards the outer zone. The zones are arranged at a fixed pitch of about several tens of micrometers from the inner periphery towards the outer periphery. Small-size Fresnel lenses are usually manufactured by plastic injection molding, and Japanese Patent Laid-Open No. 6-892 proposes a method for forming Fresnel lenses by injection molding. Since injection molding requires shorter cycle time than compression molding, productivity is significantly enhanced. However, since the base material greatly contracts during cooling, the micro-zonal structure on the Fresnel lens surface becomes wedged into the mold, making it difficult to remove the Fresnel lens from the mold. In order to solve this problem, Japanese Patent Laid-Open No. 6-892 discloses a method for facilitating mold removability by setting the temperature of a movable mold higher than a stationary mold to increase the amount of contraction of the resin in the movable mold so that the resin in the movable mold is caused to warp. There is also disclosed a gate shape for avoiding welding which occurs when the resin extends along the zones.
Fresnel lenses sometimes have external shapes other than a circular shape. For example, in a rectangular Fresnel lens, zones in an inner area of the rectangle have a 360° circular shape, whereas zones in an outer area are segmented or divided due to the rectangular shape. In an injection mold used for molding such a Fresnel lens, a section that corresponds to the circular zones have circular grooves with no openings, whereas a section that corresponds to the segmented or divided zones have circular-arc grooves that extend discontinuously to the edges of the mold.
When a Fresnel lens is to be formed by injection molding using such an injection mold, it is difficult to form all of the zones uniformly. Specifically, during a filling process in injection molding, resin is injected into a mold so that it pushes air out of a cavity. In injection molding of a Fresnel lens, the resin filling process is impeded by air in the circular grooves since they have no escape routes for air, whereas the resin filling process is performed smoothly in the circular-arc grooves since they have open ends from which air is discharged as resin is injected. Such a variation in molding characteristics results in a difference in shape between inner zones and outer zones. This will be described below in detail.
For example, FIGS. 10A and 10B illustrate a Fresnel lens having a rectangular shape. The center of the rectangle and the center of the Fresnel zones are aligned with each other. A zone 1 located on the inner side of a zone 2, which is an inscribed circle of the rectangle, has a 360° circular shape, whereas a zone 3 located on the outer side of the zone 2 is divided into two circular-arc zone segments whose central angle is smaller than 180° (see FIG. 10A). On the outer side of the zone 3 is a zone 4 located on the inner side of a zone 5 that is in contact with the short sides of the rectangle. The zone 4 is divided into two circular-arc zone segments having the same radius. A zone 6 on the outer side of the zone 5 is divided into four zone segments having the same radius (see FIG. 10B).
Referring to FIGS. 11A to 11C, when the center of the Fresnel zones is slightly shifted upward relative to the rectangle, the position where the zone shape changes from a circular shape to a circular-arc shape and the position where the number of zone segments with the same radius changes are also shifted. In this case, the aforementioned positions include the following three positions. Specifically, a zone 7 located on the inner side of a zone 8 that is in contact with the long side closer to the center of the Fresnel zones has a 360° circular shape, whereas a zone 9 on the outer side of the zone 8 has a circular-arc shape whose central angle is smaller than 360° (see FIG. 11A). A zone 11 located on the outer side of a zone 10 that is in contact with the long side distant from the center of the Fresnel zones includes two circular-arc zone segments having the same radius (see FIG. 11B). A zone 13 located on the outer side of a zone 12 that is in contact with the short sides includes four circular-arc zone segments having the same radius (see FIG. 11C).
When such a Fresnel lens is formed by injection molding, a boundary area where the zone shape or the number of zone segments changes becomes an area with different molding characteristics on the Fresnel surface. Because this boundary area appears different from other areas, an improvement in appearance quality is in demand.